linux-kernel - Re: [PATCH] xhci: clear port_remote

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Message-ID: <9286ec05-d2a2-bd23-3397-b6a3623cad8f@linux.intel.com>
Date:   Tue, 4 Jun 2019 16:53:37 +0300
From:   Mathias Nyman <mathias.nyman@...ux.intel.com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Mathias Nyman <mathias.nyman@...el.com>
Cc:     oneukum@...e.com, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xhci: clear port_remote_wakeup after resume failure

On 27.5.2019 14.28, Nicolas Saenz Julienne wrote:
> Hi Matthias,
> thanks for the review.
> 
> On Mon, 2019-05-27 at 14:16 +0300, Mathias Nyman wrote:
>> On 24.5.2019 17.52, Nicolas Saenz Julienne wrote:
>>> This was seen on a Dell Precision 5520 using it's WD15 dock. The dock's
>>> Ethernet device interfaces with the laptop through one of it's USB3
>>> ports. While idle, the Ethernet device and HCD are suspended by runtime
>>> PM, being the only device connected on the bus. Then, both are resumed on
>>> behalf of the Ethernet device, which has remote wake-up capabilities.
>>>
>>> The Ethernet device was observed to randomly disconnect from the USB
>>> port shortly after submitting it's remote wake-up request. Probably a
>>> weird timing issue yet to be investigated. This causes runtime PM to
>>> busyloop causing some tangible CPU load. The reason is the port gets
>>> stuck in the middle of a remote wake-up operation, waiting for the
>>> device to switch to U0. This never happens, leaving "port_remote_wakeup"
>>> enabled, and automatically triggering a failure on any further suspend
>>> operation.
>>>
>>> This patch clears "port_remote_wakeup" upon detecting a device with a
>>> wrong resuming port state (see Table 4-9 in 4.15.2.3). Making sure the
>>> above mentioned situation doesn't trigger a PM busyloop.
>>>
>>
>> There was a similar case where the USB3 link had transitioned to a
>> lower power U1 or U2 state after resume, before driver read the state,
>> leaving port_remote_wakeup flag uncleared.
>>
>> This was fixed in 5.1 kernel by commit:
>>
>> 6cbcf59 xhci: Fix port resume done detection for SS ports with LPM enable
>>
>> Can you check if you have it?
>> It should be in recent stable releases as well.
> 
> I was aware of that patch, unfortunately it doesn't address the same issue. In
> my case I never get a second port status event (so no PLC == 1 or any state
> change seen in PLS). The device simply disconnects from the bus.
> 

I see, ok, then we need to clear the flag in the hub thread.

But to me it looks like this patch could cause a small race risk in the successful
device initiated resume cases.

If the hub thread, i.e. the get_port_status() function, notices the U0 state before
the interrupt handler, i.e. handle_port_status() function, then port_remote_wakeup
flag is cleared in the hub thread and the wakeup notification is never called from
handle_port_status().

Would it be enough to just check for (port_remote_wakeup flag && !PORT_CONNECT) in the hub thread?
USB3 PORT_CONNECT bit is lost in most error cases.

-Mathias